The present invention relates generally to wireless transmitters and more specifically to a method and system for self-calibrating output power of a wireless transmitter.
It is often desirable that wireless transmitters, such as 802.11 capable radios used in a wireless local area network (WLAN), have calibrated transmit power levels.
Typically, calibration is performed during manufacturing. Traditional calibration methods require significant time and money, affecting both throughput on the manufacturing line and profit margins. Furthermore, radio transmit power can change over time, due to component aging, etc. These power changes can have a detrimental effect on WLANs, reducing throughput and creating a need for site maintenance, for example a new site survey, relocating or adjusting transmit power, etc. This can be particularly problematic in installations with radios in difficult to access areas, e.g., isolated location, difficult to reach locations such as a ceiling of a warehouse, etc.
Transmit power control for a typical 802.11 radio is closed loop system consisting of one or more output gain stages (with coarse and fine adjustments), and a power detector feedback circuit that outputs a value representative of the transmit power being delivered. The radio maintains a proper output power by continually monitoring the detector value during transmission, adjusting the gain stages as needed to keep the detector value equal to some desired detector “target value.” As such, calibration needs to be performed on each radio, obtaining a detector target value for each frequency at each of the various transmit power levels. These values are stored in Transmit Power Calibration Tables in non-volatile memory of the radio.
Existing calibration systems use an iterative, time consuming process to determine the detector values for a given radio. A typical system consists of a host computer that controls the process, equipment for measuring transmit power, and the device under test (the radio being tested). The process for determining the proper detector values consists of the host computer commanding the radio to transmit an open loop at a specified frequency, reading the measured power from the test equipment, adjusting the gain settings on the radio and repeating until the measured power equals the desired level. The detector value is then read from the radio by the host and stored with corresponding gain settings. This process then repeats itself for all supported frequencies, at all supported power levels. The accumulated detector and gain values are then stored into the power tables. The power tables are then transferred to the radio, often using a very slow link, and then stored in the radio's non-volatile memory.